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From: ornitz@kodak.UUCP
Newsgroups: sci.electronics
Subject: Re: Higher accuracy Ultrasonic Rangefinding
Message-ID: <759@kodak.UUCP>
Date: Mon, 23-Mar-87 16:23:48 EST
Article-I.D.: kodak.759
Posted: Mon Mar 23 16:23:48 1987
Date-Received: Wed, 25-Mar-87 02:44:38 EST
References: <1001@speech1.cs.cmu.edu> <3590001@hpfcrj.HP.COM>
Reply-To: ornitz@kodak.UUCP (barry ornitz)
Organization: Eastman Kodak Co, ECD Research Labs, Kingsport, TN
Lines: 19
Keywords: sonic velocity, heat capacities, humidity
Summary: Humidity affects sonic velocity

In article <3590001@hpfcrj.HP.COM> guest@hpfcrj.HP.COM (guest) writes:
>Response to ultrasonic transduser accuracy limits:
>The most serious limiters of accuracy in ultrasonic rangefining are WIND
>(or any type of air motion of which there is a lot) and temperature. The 
>speed of sound in air does not depend on air pressure as it is assosicated
>only with the thermal velocity of the molecules not their mean free paths.

This statement is a good approximation for air at normal atmospheric conditions
(i.e. temperatures and pressures normally found on the surface of the earth).
It also is based on an ideal gas and the assumptions that the velocity of the
medium is small (no wind) and the sound is a simple harmonic compression wave.
Large disturbances such as shock waves propagate at much higher velocities
[Hirschfelder, Curtiss & Bird:  Molecular Theory of Gases and Liquids]. With
these assumptions in mind, the additional major cause of error is the 
assumption that the composition of air is constant.  This is not the case with
changes in humidity.  The Handbook of Chemistry and Physics gives equations
for the velocity of low-frequency sound waves with humid air with varying
temperatures.
                                       Barry